This study is a continuation of our research aimed at characterizing in the primate the relationship between pain and temperature primary afferents and spinothalamic and spinobulbar neurons, and to study the roles which intraspinal and supraspinal neurons may play as modulators of these connections. Our objectives is to provide a structural and neurochemical description of pain relay mechanisms which can be correlated with physiological and pharmological events in pain sensation. We are combining immunohistochemical methods with horseradish peroxidase labelling techniques for both light and electronmicroscopic studies. We will focus on two spinal grey regions most closely linked with the sensory processing of noxious and thermal stimuli from the somatic and visceral periphery, the superficial dorsal horn and the region around the central canal (lamina X). We will investigate three major interrelationships: 1) interaction between identified somatic and visceral afferents and identified spinothalamic and spinobulbar neurons, 2) interaction between immunostained peptidergic systems (SP, SS, CCK, VIP) and identified sponothalamic and spinoreticular neurons, and 3) the impingment of modulating systems, including the enkephalinergic and serotonergic systems, onto identified primary afferents or spinothalamic or spinobulbar neurons. It is hoped that these step-by step analyses of the many elements involved in the complex circuitry of the spinal cord will lead to an understanding of the transmission and control of pain sensation. For these studies to be of maximal relevance for pain in humans, it is important to continue our longterm study of the primate in light of significant differences in their afferent systems from lower mammals.